The C# is not very audible. More effect than anything, for almost like a sub kick kinda slap sound. You would get the same thing from playing an F# the correct way, but in that range you can still make out notes. Just my $0.02.

Just out of curiosity, have you played in ranges this high or low before.
If not you should really give it a try, get all the strings and put half of them on your bass at a time just to see what you like using.
I personally do not care to use anything pass a high F-string, love to hear other people do it, just feels odd for me. I don't think I can go too low.

The C# is not very audible. More effect than anything, for almost like a sub kick kinda slap sound. You would get the same thing from playing an F# the correct way, but in that range you can still make out notes. Just my $0.02.

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I have absolutely no idea if what you've said is correct or not, but I find my B string sufficient, and I've never felt the need for a low F#, much less anything lower (while respecting those that do). To be honest, I can't even imagine notes much lower than F#, although without hearing it myself, my mind remains open.

Still, I'm going to go out on a limb and say that a high Eb would be much more musically useful. At least to me.

That's what I was leaning towards as well, also, as far as applications for the low F#, you'd need it to accompany an eight string guitar, and when given the low C#, you then have notes an entire octave LOWER than standard 4 string basses, so u can play sub-bass parts to songs, which is cool

A practical question: do you have the hands to take full advantage of that many strings? Not saying this as a "hater", more as a "been there done that".

I had 7 string envy for a particular Benavente, and was lucky enough to be able to buy it; but I quickly found out my hands aren't big enough to play low notes comfortably on such a wide neck. A narrower spacing would have made it more practical for me.

Jean Baudin plays 11 string - with a two-handed tap technique. If that's what you're aiming for, great. If you plan to play it like a bass, you'd better have hands like Kareem Abdul-Jabbar (showing my age here!).

I'd suggest you find someone with a bass like the one you would have built and give it a try before you spend money on a custom you won't be able to play.